In an electrophotographic technology, an electrostatic latent image is formed on a photoconductive member made of a photoconductive material by various methods, the electrostatic latent image is developed with a toner for forming a visible image, and then after transferring the visible toner image onto a transfer medium such as paper or OHP film, the transferred toner image is fixed to the transfer medium by any of various methods such as heating and pressing thereby to obtain a print.
Conventionally, a toner requires excellent image-reproducibility (thin lines and fine points can be accurately reproduced at developing), low-temperature fixability, hot-offset resistance (a printed paper is not soiled by toner which remain on a hot pressing fixing roller) and the like.
Today, from an environmental viewpoint, it is required to reduce remaining volatile organic compounds (VOC) in the toners. On the other hand, an image forming apparatus using toners tends to be used under high temperature and high humidity areas. Accordingly, it is required for the toners to have excellent shelf stability and durability and also to form images with a stable image density even under such environment.
Conventionally, a pulverized toner has been mainly employed. The pulverized toner is produced in such a manner that a thermoplastic resin including a colorant, a parting agent, a charge control agent and the like is melt-blended to be uniformly dispersed, the dispersion is pulverized into fine particles by a pulverizing mill, and then the fine pulverized particles are classified by a classification apparatus.
However, in the pulverized toner produced by the aforesaid pulverization method, since the parting agent and the charge control resin dispersed in the binder resin are exposed on the surface of the toner, the melted toner is easily adhered to a surface of a high-temperature pressing roll. In other words, hot-offset easily occurs. In addition, the pulverized toner has a problem in decreasing of shelf stability and environmental durability. Besides, in a toner produced by a pulverization method, because of its irregular shape, a charge amount of the toner easily fluctuated, resulting in deteriorating image-reproducibility.
Especially, a color toner includes organic pigment as a colorant. The organic pigment is easily charged and therefore is sensitive to environmental condition, so that a charge amount of the toner may easily vary. In a full color image forming method, four color toners are overprinted for forming an image. So, if only one kind of the toners may have an unstable charge amount, an image cannot be reproduced correctly. Accordingly, a toner which is not subject to by environmental conditions, particularly a toner which has a stable charge amount under a high temperature and high humidity condition, has been required.
In order to achieve such requirement, a toner producing method by various types of polymerization methods including a suspension polymerization method has been proposed. For instance, in the suspension polymerization method, a polymerizable monomer, a colorant and a polymerization initiator, and, if necessary, a crosslinkable agent, a charge control agent and other additives are uniformly melt or dispersed to form a monomer composition and then the monomer composition is polymerized to obtain a toner particle having a desired particle diameter. By producing a toner by the polymerization method, the toner has a relatively narrow particle diameter distribution and contains a parting agent and a charge control agent enveloped within a particle, whereby a toner having a stable charge amount even under a high temperature and high humidity condition will be obtained.
As an exemplary polymerization toner, a toner for developing electrostatic latent images, which comprises a particle containing at least a binder resin, a colorant and a wax, is disclosed in Patent Literature 1. The toner disclosed in the literature has a number average particle diameter in the range from 2 to 6 μm, an average circularity in the range from 0.97 to 0.995 and an amount of residual monomer of 500 ppm or less. And, observation of the cross section of the toner using a transmission electron microscope (TEM) shows that the wax components are dispersed in the binder resin in islet form. The literature demonstrates that the toner has excellent dot-reproducibility (image-reproducibility) and can form a high-quality image for a long period. However, the toner has problems such as frequent occurrence of hot-offset and low environmental durability.
Beside, Patent Literature 2 disclose a producing method of a toner, in which 100 parts by weight of a charge control resin, 10 to 200 parts by weight of a colorant and 1 to 60 parts by weight of inorganic particles are mixed to prepare a charge control resin compound, the charge control resin compound is melted or dispersed into a polymerizable monomer to obtain a polymerizable monomer compound, and then the polymerizable monomer compound is polymerized in an aqueous dispersion medium. The toner produced according to the literature has excellent spectral property, such as translucency, necessary for reproducing a color tone of a color image clearly. And, by using the toner, an image can be formed with a high image density without generating fog. However, the toner requires improving stability of an image density under a high temperature and high humidity condition.
Published Patent literature 1: Japanese Patent Application Laid-open Hei 11-344829,
Published Patent literature 2: Japanese Patent Application Laid-open 2003-131428.